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A139268
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Twice nonagonal numbers (or twice 9-gonal numbers): a(n) = n(7n-5).
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3
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0, 2, 18, 48, 92, 150, 222, 308, 408, 522, 650, 792, 948, 1118, 1302, 1500, 1712, 1938, 2178, 2432, 2700, 2982, 3278, 3588, 3912, 4250, 4602, 4968, 5348, 5742, 6150, 6572, 7008, 7458, 7922, 8400, 8892, 9398, 9918, 10452, 11000
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = A001106(n)*2 = 7*n^2 - 5*n = n*(7*n-5).
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MATHEMATICA
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2*PolygonalNumber[9, Range[0, 40]] (* or *) LinearRecurrence[{3, -3, 1}, {0, 2, 18}, 50] (* Harvey P. Dale, Feb 08 2024 *)
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PROG
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CROSSREFS
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Cf. numbers of the form n*(n*k-k+4))/2 listed in A226488 (this sequence is the case k=14). - Bruno Berselli, Jun 10 2013
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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